The present invention relates to a filling device for and a method of filling balls, in particular solder balls, in the apertures of a ball-receiving element, in particular a mask or template, for application as a ball grid array.
With the ever increasing integration of electronic chips, ball grid array packages are finding increasing application as, for example, high density input/output chip carriers.
A known filling device for filling solder balls in the apertures of a mask or template for application on a substrate as a ball grid array is, for example, disclosed in U.S. Pat. No. 5,655,704.
Such a filling device, as represented schematically in FIG. 1, comprises a container 1 which includes an opening 2 at a lower surface thereof and contains a supply of solder balls 3. The container 1 is movably disposed over the upper surface of a ball-receiving element 5 which includes an array of apertures 7 defining the pattern of the required ball grid array. The ball-receiving element 5 is located above a substrate 9, typically of a ceramic or plastics material, which includes an array of contact pads 11 each having an adhesive layer 13, typically of a flux, provided thereon for retaining a solder ball 3 prior to reflow of the solder. In use, a substrate 9 is located beneath the ball-receiving element 5 such that the contact pads 11 thereon are brought into registration with the apertures 7 in the ball-receiving element 5. The container 1 is then moved (as indicated by arrow A) over the region of the ball-receiving element 5 which includes the array of apertures 7 such that solder balls 3 can fill the apertures 7. With the container 1 remote from the array of apertures 7, the substrate 9 is removed from beneath the ball-receiving element 5, with the solder balls 3 being held in position by the adhesive layers 13, and then heated in an oven to cause reflow of the solder.
Whilst such a filling device is functional, there are a number of problems associated with this filling device which reduce the filling efficiency. One significant problem so arises from the need to maintain a large quantity of solder balls in the container in order to ensure a supply of solder balls over the entire area of the opening in the container. The provision of such a large quantity of solder balls, as a plurality of layers, leads to forces having a relatively high horizontal component between adjacent solder balls, the solder balls and the container and the solder balls and the ball-receiving element during movement of the container.
The existence of those forces significantly reduces the filling efficiency of the filling device as the horizontal component of those forces acts to restrain the solder balls from falling freely under the influence of gravity into the apertures. As will be appreciated, any restraint to the vertical movement of the solder balls decreases the filling efficiency as the time period in which a solder ball and an aperture are sufficiently in registration to allow the solder ball to enter the aperture is very short, typically of the order of a few milliseconds. Indeed, with such a filling device it is not uncommon, particularly at sweeping speeds of greater than 10 mm/s, for many apertures to be empty after sweeping the filling device over the ball-receiving element. Such a poor filling efficiency is highly undesirable for the reason that to achieve complete filling either several sweeps of the filling device at a relatively low speed would be necessary or alternatively a subsequent expensive and delicate repair operation would be necessary. In addition, incomplete filling of the apertures is undesirable because when empty apertures are reached by the rear edge of the container, there is a real risk, as illustrated in FIG. 2, of solder balls being partly engaged in those empty apertures and sheared by the rear edge of the container, which sheared solder balls can subsequently enter apertures and prevent the required filling by complete solder balls as is necessary to achieve the required tolerances of the solder bumps of fabricated ball grid array packages, and also create debris which hinders the operation of the filling device.
The existence of these forces further reduces the filling efficiency of the filling device and impairs the condition of the solder balls in developing relatively high stresses at the surfaces of the solder balls which are sufficient to dull and even deform the solder balls, particularly when the container is moved at high speed.
It is thus an aim of the present invention to provide a filling device and a method which provides for the improved filling of balls in an array of apertures in a ball-receiving element.
Accordingly, the present invention provides a filling device for filling balls in an array of apertures in a ball-receiving element, the filling device comprising: a housing including an opening at a lower surface thereof and defining in part a chamber for containing a supply of balls, the housing being in use movably disposed over a ball-receiving element including an array of apertures; and distribution means disposed within the housing for distributing balls contained in the chamber such as to maintain a limited number of layers of the balls over at least a region of the opening in the housing.
Preferably, the distribution means is configured to provide substantially a single layer of balls over at least a region of the opening in the housing.
In one embodiment the distribution means comprises a track-defining member having a lower surface operably disposed in spaced relation to the lower surface of the housing such as to define a track of restricted height therebeneath when disposed over the ball-receiving element.
Preferably, the lower surface of the track-defining member is substantially planar.
Preferably, at least the lower surface of the track-defining member is formed of a resilient material.
Preferably, the track-defining member is spaced relative to the lower surface of the housing such as to define a track for a single layer of balls therebeneath.
In one preferred embodiment the track-defining member is spaced relative to the lower surface of the housing such as to define a channel having a height substantially equal to the diameter of the balls.
In another preferred embodiment at least the lower surface of the track-defining member is formed of a resilient material and the lower surface of the track-defining member is spaced relative to the lower surface of the housing such as to define a channel having a height less than the diameter of the balls, whereby balls are biased downwardly when located beneath the track-defining member.
Preferably, the track-defining member is movably disposed relative to the housing, and the filling device further comprises: a loading element biasing the track-defining member downwardly relative to the lower surface of the housing.
More preferably, the biasing force of the loading element is such as to maintain a single layer of balls therebeneath.
Preferably, the filling device further comprises: a reservoir for containing a supply of balls; and at least one channel connecting the reservoir to the chamber.
More preferably, the lower end of the at least one channel is spaced from the lower surface of the housing such as to restrict the fill level of balls in the chamber.
More preferably, the filling device comprises at least one channel adjacent a forward edge of the track-defining member in a direction of movement.
Yet more preferably, the filling device comprises at least one channel adjacent each of forward and rearward edges of the track-defining member in a direction of movement.
More preferably, the filling device comprises a plurality of channels.
In another embodiment the distribution means comprises at least one channel through which balls are in use supplied to the chamber, the lower end of the at least one channel being spaced from the lower surface of the housing such as to restrict the fill level of balls at at least a region of the opening in the housing.
Preferably, the filling device comprises a plurality of channels.
In a further embodiment the distribution means comprises first and second downwardly-extending members which define an enclosed space therebeneath, the downwardly-extending members being configured to allow only a limited number of layers of balls to pass therebeneath.
In one preferred embodiment the first and second downwardly-extending members are laterally spaced.
In another preferred embodiment the first and second downwardly-extending members are downwardly and outwardly flared.
Preferably, the downwardly-extending members are formed of a resilient material.
In a yet further embodiment the distribution means comprises at least one projection extending inwardly of the housing and including a surface having a forward and downward component in a direction of movement.
In one preferred embodiment the at least one surface is a substantially planar surface.
In another preferred embodiment the at least one surface is a curved surface.
Preferably, the distribution means comprises first and second projections, each extending inwardly and including a surface having a forward and downward component in the respective directions of movement.
Preferably, the housing includes a wiper disposed internally along one, rearward edge thereof in a direction of movement.
More preferably, the housing includes wipers disposed internally along each of the edges thereof in the respective directions of movement.
Preferably, the balls comprise solder balls.
The present invention also provides a ball bumping apparatus, comprising: a ball bumping station comprising the above-described filling device; and a ball-receiving element including an array of apertures over which the filling device is movably disposed.
In one preferred embodiment the ball-receiving element comprises a mask including an array of apertures.
In another preferred embodiment the ball-receiving element comprises a template including an array of apertures.
Preferably, the ball bumping apparatus further comprises: a stencil printing station for printing an array of ball-retaining deposits on a substrate, the stencil printing station comprising a stencil including an array of apertures having the same pattern as the array of apertures in the ball-receiving element, and a print head for delivering a ball-retaining material to the surface of the stencil.
More preferably, the filling device and the print head are configured to be operable in unison such as to print ball-retaining deposits on one substrate at the same time as providing a ball grid array on another substrate.
The present invention further provides a ball bumping apparatus comprising a stencil printing machine having the above-described filling device fitted thereto. One suitable stencil printing machine is the DEK 265 screen printing machine as manufactured by DEK Printing Machines Ltd.
The present invention yet further provides a method of filling balls in an array of apertures in a ball-receiving element, comprising the steps of: providing a filling device over a ball-receiving element including a plurality of apertures, the filling device comprising a housing including an opening at a lower surface thereof and defining in part a chamber containing a supply of balls, and distribution means disposed within the housing for distributing the balls contained in the chamber such as to maintain a limited number of layers of the balls over at least a region of the opening in the housing; and moving the filling device relative to the ball-receiving element such as to cause the filling of the apertures in the ball-receiving element.
The present invention still yet further provides a filling device for filling balls in an array of apertures in a ball-receiving element, the filling device comprising: a housing including an opening at a lower surface thereof and defining in part a chamber for containing a supply of balls, the housing being in use movably disposed over a ball-receiving element including an array of apertures; and distribution means disposed within the housing for causing circulation of the balls contained in the chamber over at least a region of the opening in the housing.
The present invention yet also further provides a filling device for filling balls in an array of apertures in a ball-receiving element, the filling device comprising: a housing movably disposed in use over a ball-receiving element including an array of apertures, wherein the housing defines at least in part a chamber for containing a supply of balls and includes at least first and second relatively movable parts movable between a first, open configuration in which an opening is defined at a lower surface thereof in communication with the chamber and a second, closed configuration in which the chamber is closed.
Preferably, the first and second movable parts are coupled by a threaded component which on rotation causes relative movement thereof.
More preferably, the first and second movable parts comprise the forward and rearward edges of the housing in a direction of movement.
The present invention yet still also further provides a ball bumping apparatus, comprising: a ball bumping station comprising a filling device for filling balls in an array of apertures in a ball-receiving element, the filling device comprising a housing including an opening it a lower surface thereof and defining in part a chamber for containing a supply of balls, and a ball-receiving element including an array of aperture s over which the filling device is in use movably disposed; and a stencil printing station for printing an array of ball-retaining deposits on a substrate, the stencil printing station comprising a stencil including an array of apertures having the same pattern as the array of apertures in the ball-receiving element, and a print head for delivering a ball-retaining material to the surface of the stencil.